The mechanism of the rearrangement of N-(1-arylalkylidene)cyanomethylamines ArC(NCH₂CN)R 1 to the corresponding nitriles ArCH(CN)R 2 (in DMF, at 150 °C, with K₂CO₃) is described. Reaction 1 → 2 was investigated for different types of imine 1, and it was found that with a leaving group other than CN⁻ the reaction does not proceed to yield the nitrile, whereas imines such as PhC(NCH₂CN)H, prepared starting from aldehydes rather than ketones, yield the expected phenylacetonitrile even at temperatures as low as 120 °C. Evidence for the mechanism comes from a study of the reactivity of the postulated intermediates: 2-cyano-3-phenylaziridine 4c, and 2,2-diphenyl-2H-azirine 5b. The route involving aziridine 4c is ruled out, since this compound does not react at all under the investigated conditions. The 2H-azirine 5b instead, yields the corresponding diphenylacetonitrile in DMF with K₂CO₃, at 150 °C. The transformation seems to involve an initial deprotonation, followed by an intramolecular ring closure–CN elimination step, which yields the 2H-azirine. The azirine then isomerizes to the nitrile. Additional evidence for the intermediacy of the 2H-azirine, based on ¹H NMR monitoring of the reaction 1 → 2, is described. Finally, the results of a simple isotope exchange experiment provide a rationale for the previously observed scrambling of labels, and further confirm the proposed mechanism.